This invention relates to battery separators for retained-electrolyte maintenance-free lead-acid rechargeable batteries, which have a high affinity for the electrolyte whereby it is tightly held in the separator matrix during use of the battery.
McClelland et al. U.S. Pat. No. 3,862,861 teaches a new class of sealed lead-acid batteries. Unique performance characteristics are obtained by employing an ultrafine fiber diameter absorptive mat of large surface area per unit of volume and a large porosity, enabling retention of the bulk of the acid electrolyte (capacity determining) of the cell in the separator phase while leaving a sufficiently thin layer of electrolyte on the plate particle surfaces and pore walls to sustain internal oxygen recombination within the cell at high efficiencies, even at elevated rates of overcharge.
In commercial practice, the cells have employed one or more identical separator layers formed of a mixture of glass fibers, the bulk of which are in the 0.1-1.0 micron range, with lesser amounts in the 1.0-5 micron range and, for structural and handling requirements minor amounts of fibers in the 5-20 micron range. These ultrafine glass fiber separators serve well in this application because of the high heat of wetting of sulfuric acid on glass, their large surface area per unit of volume of separator material (approximately 0.1 to 20 square meters per gram of silica), and porosities ranging as high as from about 85-95 percent. Typical separator materials of this type are illustrated in published U.K. Patent Application No. 2,048,556A (see scanning electron photomicrographs of FIGS. 5 and 6).
Multilayered separators for maintenance-free lead-acid cells in which at least one of the layers includes a highly retentive ultrafine fiber diameter glass material of the type previously discussed, are also known. For instance, see Eisenacher et al. U.S. Pat. No. 3,753,784, Kono et al. U.S. Pat No. 4,262,068 (see particularly column 1, lines 58 through column 2, line 9), McClelland U.S. Pat. No. 4,137,377 and Japanese Application No. 79/98,386 (Chemical Abstracts 94:195041k).
It is an object of this invention to define a multilayered separator for sealed lead-acid batteries which enhances the performance of the battery particularly at high rates of discharge and/or discharge at low temperature.